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- Dilation (4) (entfernen)
Detailed atomistic simulations were carried out for swelling polymer/gas systems related to experimental sorption and dilation data for CO2 and CH4 in three glassy polymers (polysulfone PSU, the polyimide 6FDA-TrMPD, and a polymer of intrinsic microporosity PIM-1) at 308 K (35 °C) and pressures up to 50 bar. Corresponding experiments were performed with a gravimetric sorption balance and a dilatometer based on a capacitance distance sensor. For each polymer/gas system molecular packing models were prepared and equilibrated for two reference states: the pure polymer is taken as reference for the respective 'unswollen' state and similarly the state of the highest penetrant pressure reached in the corresponding experiment is taken to represent the 'swollen' state. Models for the latter were constructed in agreement with experimental data (pressure, temperature, gas concentration and volume dilation). Concentration–pressure isotherms of each polymer/gas system were obtained using Grand Canonical Monte Carlo (GCMC) simulations for both reference states (depleted of gas molecules), which are in good agreement with the experimental data in the respective pressure range. As expected these isotherms – due to the simulation technique used, merely based on hole-filling in a static host matrix – do not represent the sorption behavior over a broader range of gas pressures which may involve significant structural rearrangements as well as swelling and relaxational phenomena. Nevertheless, a linear combination of the two GCMC-isotherms allows the interpolation in order to describe the nonlinear gas sorption in the glassy polymers under investigation covering the penetrant pressure range between the reference states in good agreement with the experimental results.
A molecular modeling investigation of dilation effects induced by sorbed gas molecules in two glassy polymers is presented. As experimental reference, integral sorption of CO2 and CH4 was measured for polysulfone (PSU) and a polyimide (6FDA-TrMPD, PI4) at 308 K and a pressure of 10 bar. Simultaneously, the gas induced swelling effect was measured with a dilatometer based on a capacitive distance sensor recorded. The experimental evidence of the (on the observed time scale and concentration levels) elastic nature of the gas induced dilation is supported by the dilation and contraction behavior observed in molecular dynamics (MD) simulations of respective detailed atomistic packing models. These models were constructed in accordance with gas concentration levels obtained from the experimental sorption results. Quantitative deviations between simulated and measured dilations are discussed as a consequence of an anelastic response of the polymer matrix which is too fast to be resolved in the experiments whose kinetics is dominated by diffusional processes. In the simulation, the initial insertion of penetrant molecules into equilibrated packing models circumvents the slow diffusional process of the experiment and allows a reasonable representation of the dilation process as well as a closer investigation. Our simulation approach reveals a different behavior for PSU and PI4 on the corresponding time scale. Most likely, the different chain mobility of the two polymers is responsible for the respective response to the inserted amount of gas molecules which is discussed in terms of the different chain mobilities of the two polymers.
Experimental sorption and dilation data of the polysulfone/CO2 system at 308 K and pressures up to 50 bar were measured utilizing a gravimetric sorption balance and a dilatometer based on a capacitive distance sensor. The data of this glassy polymer/gas system were subjected to a thorough kinetic analysis on the basis of a viscoelastic model, which allows the separation of the diffusive/elastic fraction of the sorption/dilation process from the slower relaxational part. The results were interpreted in terms of the common dual mode sorption model and the site distribution model of Kirchheim. Detailed atomistic packing models of the same polymer/gas system were created for two reference states with regard to concentration and swelling. The CO2 sorption isotherms of the packing models corresponding to these two swelling states, calculated using GCMC simulations, could be combined in order to interpolate the gas uptake over the intermediate pressure range with good agreement to the experimental data. The elastic part of the gas induced dilation is successfully described by MD simulations and derived partial molar volumes are in satisfying agreement with experimental findings. Finally, the free volume of the packing models is probed and the obtained size distribution of the free volume elements is compared to the results of the analysis of experimental data according to the site distribution model.